This invention relates to ice detection and de-icing systems, and more particularly to ice detection and de-icing systems for use on the leading edge of an aircraft wing.
Ice accretion, or buildup, on the surface of aircraft can have deleterious effects on flight performance. Lift decreases, thrust falls off, drag and weight increase, and the stall speed rises sharply. Indeed, sometimes the effects are serious enough to cause a crash. A reliable ice detection system is thus required to detect the onset of ice accretion. Such a system allows either for changing the flight path of the aircraft and getting it out of the icing environment, or for activating a de-icing system to remove the ice and enabling the aircraft to continue on a desired course.
Recently developed technology has made some advances from such imprecise methods as relying on weather forecasts, ambient temperature readings, and visual cloud sightings.
The ice detection system disclosed by Daniels in U.S. Pat. No. 4,775,118 uses an Electro-Impulse De-Icing (EIDI) coil to induce eddy currents in an aircraft surface skin. An accelerometer senses vibrations in the skin. The presence of ice changes the manner in which the skin responds to the electromagnetic forces imparted by the coil. Thus the accelerometer can sense the presence of ice.
The ice detection system disclosed by Daniels has certain limitations and disadvantages. First, the accelerometer is a point sensor, as opposed to a distributed sensor system, in which the sensor can monitor a large area. Second, the accelerometer is too large to be embedded in a skin or surface, and therefore it must be attached to the skin. Third, as a power source is required to pulse the EIDI coil, Daniels's system is necessarily active, not passive. Fourth, the Daniels system is not a self-contained one, which can be easily maintained or replaced if necessary. A self-controlled system, especially a passive one, would be well suited to retrofitting existing aircraft. And fifth, the point sensing technique disclosed by Daniels is prone to misinterpretation under certain icing conditions. Specifically, an increase in mass on the surface will decrease the frequency of the surface vibrations, while an increase in surface stiffness will increase the frequency. If the increase and the decrease cancel one another, the icing condition will not be detected properly with the teachings of Daniels.
Daniels also incorporates a method for removing ice by pulsing the surface with the EIDI coil at a higher voltage than for producing surface waves for ice detection. That method was known in the art.
In U.S. Pat. No. 4,732,351, Bird discloses an ice detection system using a piezoelectric material to cover a surface. While this is a distributed system, Bird does not teach a nonintrusive embedding of the piezoelectric material, an autonomous system, or a passive one. Bird teaches removing ice by inducing waves in the piezoelectric covering material.
One ice removal system which has proven effective is described by Haslim et al. in U.S. Pat. No. 4,690,353. Goldberg et al. purport to teach improvements of that patent in "Developments in Expulsive Separation Ice Protection Blankets". These references describe an electro-expulsive separation system in which an elastomeric covering referred to as a blanket, cuff, or boot is placed on an aircraft surface. Mutually repelling conductors are employed to distend the blanket abruptly, thus propelling ice off the protected surface. It would be desirable to combine this ice removal system with an effective ice detection system such that the entire system is autonomous, minimally intrusive, and easily retrofittable.
One concept of "smart structures" or "smart skins", in which sensors, actuators, and microprocessors are integrated, has been disclosed by Heyman et al. in "Fiber Optic Sensor Technology--An Opportunity for Smart Aerospace Structures", AIAA-88-4665 (AIAA/NASA/AFWAL Conference on Sensors and Measurements Techniques for Aeronautical Applications, Sep. 7-9, 1988). Heyman et al. disclose using fiber optics as sensors. Disadvantages of using fiber optics include not being easily retrofittable and not working well in elastomers due to fiber stiffness.